This invention concerns the processing of video signals encoded according to the standard adopted by the Moving Picture Experts Group (MPEG) and in particular, a method of splicing an MPEG encoded data stream from a first source to an MPEG encoded data stream from a second source in a manner that ensures that no video data is lost when the combined image is reproduced.
In the United States a standard has been proposed for digitally encoded high definition television signals. This standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Standards Organization (ISO). This standard is described in a draft international standard (DIS) publication entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.262" which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 systems standard "ISO/IEC 13818-1" and for its teachings on the MPEG-2 digital video encoding standard "ISO/IEC 13818-2."
The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 "standards", known as Main Profile, Main Level is intended for encoding video signals conforming to existing television standards (i.e., NTSC and PAL). Another "standard" known as Main Profile High Level is intended for encoding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line. It is this standard which is currently being implemented as the United States HDTV standard.
The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable the signals having several different formats to be covered by the standard. These formats define images, having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG-2 bit stream representing a sequence of images in six layers, the sequence layer, the group pictures layer, the picture layer, the slice layer, the macro block layer, and the block layer. Each of these layers is introduced with control information and "stuffing" characters. The stuffing characters are inserted as needed to ensure that the data rate of the input data stream matches the rate at which pictures are displayed. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zig-zag patterns and dequantization information) are interspersed throughout the coded bit stream.
To effectively receive the digital images, a decoder must recognize the control portions, extract the necessary control information, and use the extracted data to process the video signal information. One piece of information that is specified for each sequence layer is the video buffering verifier (VBV) buffer size. The VBV buffer size value specifies a number of bits of input data from the video sequence which must be stored in the input buffer of the decoder before the video sequence may be decoded. If this number of bits is stored when the decoding operation begins, the input buffer will neither become too full (overflow) or become empty (underflow) during the processing of the video sequence.
The VBV buffer size value assumes that the input buffer is empty when the new sequence data is received or, at least that all of the data in the input buffer will be removed before the contents of the input buffer reach the VBV buffer size value. To ensure that the VBV buffer size value does not cause the input buffer to overflow or underflow, it is preferred in the MPEG standard that any image splicing be done on a sequence boundary.
This may be unacceptable, however, for video editors who would like to switch from one scene to another after a particular picture has been displayed, irrespective of whether that picture is at a sequence boundary. It may also be undesirable for broadcasters who wish to insert short program segments, for example station identification messages or advertisements into an HDTV program at arbitrary intervals.
If splicing is not done at a sequence boundary then the data already resident in the input buffer may not be read from the buffer for processing until the buffer overflows due to the new data from the inserted sequence. Alternatively, the data already resident in the input buffer may cause the decoder to prematurely indicate that the VBV buffer size criterion has been met. In this instance, the decoder may cause the input buffer to either overflow or underflow in the processing of the stored data.
One method of solving this problem is proposed in the MPEG-2 standard DIS ISO/IEC 13818-1. By this method, certain access units, or pictures, in the data stream are marked with a SEAMLESS.sub.-- SPLICE.sub.-- FLAG. This flag is used to mark safe locations for splicing an inserted sequence into the main sequence. As with the conventional splicing method, however, there are only a limited number of access units which satisfy the criteria to have this flag set. Furthermore, this splicing method limits the bit rate of the inserted stream to a certain level to meet its criteria and the inserted stream can not be a variable bit rate stream.
Another method would be to define a format within the MPEG standard which allows for easy insertion and provide a flag identifying this format. This method, however, cannot handle incompatible formats and reduces the robustness of the MPEG standard.